Step motor control circuit

ABSTRACT

A permanent magnet step motor rotor is driven in response to oppositely directed magnetic fluxes coupled by a magnetic core to the rotor. The flux in one direction is derived by selectively feeding D.C. current in one direction through a winding means on the core and a transistor emitter collector path while a switch is open circuited to forward bias the transistor base emitter junction. Flux in the other direction is derived by closing the switch and thereby back biasing the transistor so that D.C. current flows in the winding means in the opposite direction. The rotor is locked in place under steady state conditions with a D.C. current derived from the circuit including the transistor and switch. The steady state current is less than the current required to drive the rotor and is maintained at the lower level by including capacitors in bias circuits for the transistor. To drive the rotor in response to changing magnetic fluxes there may be provided either shading rings, extended, segmented core pole faces, or a rotor having major and minor permanent magnet poles. To provide balanced magnetic flux in opposite magnetic core pole faces coupling flux to the rotor a low resistivity slotted ring is provided.

States Patent Koihagen [54] STEP MOTDR CONTROL CIRCUIT Walter Kolhagen,818 Oakley Ave., Elgin, 111. 60120 22 Filed: May 6,1970

21 Appl.No.:35,066

[72] Inventor:

[52] US. Cl. ..318/1138, 318/439, 318/696, 310/156, 310/172 [51] lint.Cl. ..H02k 29/00 [58] Field of Search ..310/156, 172; 318/254, 138,318/691, 685, 439

[56] References Cited UNITED STATES PATENTS 3,404,324 10/ 1 968 Gerard..318/696 3,453,514 7/1969 Raues et a1. ..318/138 3,560,829 2/ l 971Brennan ..318/599 2,870,388 l/1959 Thomas ..318/138 3,461,367 8/ 1969Takeyasu et a1 ..318/138 3,290,573 12/1966 Kamens ..318/681 3,448,3596/1969 Engel ..318/681 2,753,501 7/1956 Brailsford ..318/254 3,375,4223/1968 Bodigues ..318/254 X 3,041,487 6/1962 Hurst ..310/172 2,968,7551/1961 Baermann ..318/254 3,042,847 7/ 1962 Welch ..318/254 3,402,3379/1968 Malmborg et a1. ..318/254 3,370,189 2/1968 Haydon et al ..310/172X Primary Examiner-G. R. Simmons Att0rney-Lowe and King 1 sc'r Apermanent magnet step motor rotor is driven in response to oppositelydirected magnetic fluxes coupled by a magnetic core to the rotor. Theflux in one direction is derived by selectively feeding DC. current inone direction through a winding means on the core and a transistoremitter collector path while a switch is open circuited to forward biasthe transistor base emitter junction. Flux in the other direction isderived by closing the switch and thereby back biasing the transistor sothat DC. current flows in the winding means in the opposite direction.The rotor is locked in place under steady state conditions with a DC.current derived from the circuit including the transistor and switch.The steady state current is less than the current required to drive therotor and is maintained at the lower level by including capacitors inbias circuits for the transistor. To drive the rotor in response tochanging magnetic fluxes there may be provided either shading rings,extended, segmented core pole faces, or a rotor having major and minorpermanent magnet poles. To provide balanced magnetic flux in oppositemagnetic core pole faces coupling flux to the rotor a low resistivityslotted ring is provided.

18 Claims, 13 Drawing Figures PATENTEDMAR 1 3 I975 3, 720.864

sum 2 or a JTIUEA FKS PATENTEIJMAR 1 3 ma 3 720.864

- SHEET 3 OF 3 JTI'OZA/EYS' 1 STEP MOTOR CONTROL CIRCUIT The presentinvention relates generally to step motors having permanent magnetrotors. More particularly, in one aspect, the invention relates to apermanent magnet rotor step motor excitation circuit feeding currents toa motor excitation winding to induce oppositely directed rotor steppingand braking magnetic fluxes in a core magnetically coupled with therotor. In another aspect, the invention relates to permanent magnetrotor step motors wherein a low resistivity strap is responsive tounequal magnetic fluxes flowing through pole faces of the core toequalize the flux in the pole faces.

In the prior art there exist numerous stepping motors of the permanentmagnet rotor type wherein the rotor is stepped in response to a switchbeing opened and closed to sequentially couple a magnetic field to therotor. The prior art permanent magnet rotor step motors generallyinclude a magnetic core having either a permanent magnet orelectromagnet to brake the rotor after the rotor has been stepped inresponse to a magnetic field induced in the core by a current pulse fedto a coil on the core. The current pulse supplied to the core produces amagnetic flux of sufficient magnitude to overcome the braking effects ofthe permanent magnet, either by a bucking action, as disclosed in my US.Pat. No. 3,423,716, or by overpowering a steady state magnetic fieldproduced by either a permanent magnet or electromagnet, as disclosed inU.S. Pat. No. 3,370,189 to Haydon. Prior art step motors of the definedclass also generally include an air gap in the core thereof to maintainthe field required for braking and to provide magnetic field reluctancebalancing for an air gap existing between pole faces of the core and thepermanent magnet rotor. An air gap in the core causes ineffiency incoupling switching flux to the rotor due to the high reluctance pathcreated thereby.

In accordance with the present invention a per manent magnet rotor stepmotor is braked without the need for means to establish a DC. brakingfield with a permanent magnet or electromagnet and the requirement forcore air gaps is eliminated. A step motor of this type is achieved, inaccordance with the present invention, by employing a unique drivecircuit for the winding means of the step motor core. The drive circuitexcites the winding while the rotor is stationary to brake the rotor. Tostep the rotor, current is supplied by the drive circuit to the coil inopposite directions, to induce oppositely directed fluxes in the coreand rotor. In response to each transition of the core and rotor flux,the rotor is stepped. The drive circuit includes a voltage responsiveswitch, preferably of the solid state or transistor type, connected inseries with at least a portion of the winding. The switch includes avoltage responsive control electrode that is selectively forward andback biased to control conduction through the switch and the directionof DC current applied to the winding. The bias voltage is responsive toa control switch so that the rotor-is driven twice each time the controlswitch is opened and closed.

In accordance with one aspect of the invention a single transistor and awinding having a pair of segments control the direction of flux flow inthe magnetic core. In accordance with another arrangement, a singlewinding is connected to be responsive to alternate conductions throughpairs of complementary transistors. The arrangement employing a singletransistor and split winding segments is characterized by its simplicityand minimum number of components, while the arrangement employing pairsof complementary transistors is highly efficient as the entire windingor coil is utilized to produce magnetic fluxes in opposite directions.

A further feature of the invention is that the steady state magneticbraking flux applied to the rotor is derived in response to a relativelylow current level, while a high current level required to drive thepermanent magnet rotor is achieved only under transient conditions.Thereby, efficiency of the unit is enhanced because the current requiredfor locking the rotor in place is minimized and the required current fordriving the rotor is attained only when necessary.

In accordance with still another aspect of the invention, fluxes in polefaces of the motor core are equalized despite uneven excitation ofmagnetic fluxes in pole faces feeding flux into the rotor. This featureis achieved by providing a relatively low resistivity slotted ring inthe vicinity of the pole faces and rotor. The low resistivity'ringfunctions in a manner similar to a transformer winding to interceptleakage flux resulting from uneven flux excitation so that a current isinduced in the ring. In response to the current induced in the ring,fluxes are produced in the pole faces and the rotor does not have atendency to be torqued under steady state conditions while a constantmagnetic flux is being supplied thereto. The conducting ring enableswindings carried on legs of the magnetic core feeding fluxes through thepole faces to opposite sides of the rotor to be wound in a relativelyimprecise manner, obviating the need for the windings to have exactlythe same number of turns:

In addition, the conducting ring enables a single coil to be utilized toexcite a permanent magnet rotor of a step motor even-though the coil isdisplaced by dif-* ferent amounts from pole faces of the motor core.

In accordance with another feature of the invention, rotation of thepermanent magnet rotor can be effected in response to oppositelydirected magnetic fields without the need for flux delaying shadingrings. In accordance with one embodiment, pole faces of the motor coreare extended circumferentially about a portion of the magnetic rotor.The extended portions have a lateral dimension running parallel to therotor Ion gitudinal axis, and hence pole force area, less than thecorresponding dimension and pole face area of main segments of the polefaces. Since the. force applied to the rotor is greater while atransition is being made in the magnetic field flux direction thanduring steady state operation, the extended portions of the pole facescause rotation of the rotor. During steady state operation the flux fromthe extended portion of the pole faces is not sufficient to torque themotor which is maintained in a braked condition by virtue of the fluxflowing through the main portion of the pole faces. In accordance withanother embodiment, the rotor is provided with major and minor permanentmagnet poles. The minor permanent magnet poles enable the roto'r to bedriven in response to transient variations of the magnetic field appliedto the rotor by the core, while the major permanent poles brake therotor under steady state conditions.

It is, accordingly, an object of the present invention to provide a newand improved step motor driving circuit.

Another object of the invention is to provide a new and improved stepmotor driving circuit wherein a step motor need not include meansexternal to the drive circuit for establishing a D.C. magnetic field inthe motor core.

Still another object of the invention is to provide a step motor drivecircuit wherein the need for an air gap in the step motor magnetic coreis obviated.

A further object of the invention is to provide a drive circuit for apermanent magnet rotor step motor wherein the rotor is stepped twiceeach time a control switch is opened and closed.

An additional object of the invention is to provide a drive circuit fora permanent magnet rotor step motor utilizing a single on-off typeswitch for selectively supplying current in one direction to a coil ofthe motor and for controlling an electronic switch supplying current inthe other direction to the motor coil.

Still another object of the present is to provide a step motor drivecircuit requiring only a single transistor or other electronic switchingdevice to control the direction of current applied to the motor rotor.

Yet another object of the present invention is to provide a permanentmagnet rotor step motor braked in response to flux derived from the samewinding as that employed for deriving rotor drive flux.

Yet another object of the invention is to provide a drive circuit for apermanent magnet rotor step motor wherein a single winding is responsivethroughout its length to oppositely directed D.C. currents to maintainthe rotor in a locked position, as well as to control rotation of therotor.

A further object of the invention is to provide, in combination with astep motor having no permanent magnets or air gaps, a means for steppingthe rotor once in response to each change in the direction of a magneticfield, and wherein the need for shading rings is obviated.

Still another object of the invention is to provide a new and improvedmeans for equalizing the flux in pole faces of a magnetic core coupledto a permanent magnetic rotor of a step motor.

Yet another object of the invention is to provide a new and improvedmeans for obviating the requirement for precisely the same number ofturns to be wound on legs feeding flux into pole faces coupled to apermanent magnet rotor of a step motor.

Yet another object of the invention is to provide a new and improvedmeans for enabling a single coil to energize a permanent magnet rotorstep motor even though the reluctance of magnetic materials coupling thefield from the coil to opposite faces adjacent the rotor is different.

The above and still further objects, features and ad vantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of several specific embodiments thereof,especially when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is a circuit diagram of one embodiment of the present inventionutilizing a center tapped coil;

FIGS. 2 and 3 illustrate modifications of the basic circuit ofFIG. 1;

FIG. 4 is a circuit diagram of still another embodi-' ment of thepresent invention wherein a pair of coils are provided;

FIG. 5 is a circuit diagram of still a further embodiment of theinvention wherein a single coil is employed;

FIG. 6 is a side view of the pole faces of a modified step motorresponsive to the drive circuit of the invention;

FIG. 7 is a view taken through the lines 7-7, FIG. 6, to illustrate therelative cross-sectional areas of pole face segments utilized in theembodiment of FIG. 6;

FIG. 8 is a side view of a rotor in accordance with another embodimentof the present invention;

FIG. 9 is a plan view of a rotor of the type illustrated in FIG. 8;

FIG. 10 is a front view of still another embodiment of the inventionwherein a conducting strap is employed to equalize fluxes in opposedlegs of a magnetic core for motor;

FIG. 11 is a view taken through the lines 11l1 of FIG. 10 to provide anillustration of the manner by which a conducting strap and an iron coreare fitted together;

FIG. 12 is a perspective view of the conducting strap employed in theembodiments illustrated by FIGS. 10 and 11; and

FIG. 13 is a front view of still another embodiment employing aconductive strap and having only a single winding.

Reference is now made specifically to FIG. 1 of the drawings whereinthere is illustrated a step motor 11 having a permanent magnet rotor 12and a laminated iron core 13 having only a single air gap 14 in whichthe permanent magnet rotor is positioned. Rotor 12 is positioned betweenpole faces 15. and 16 of core 13 and preferably includes a multiplicityof permanent magnet poles, although for purposes of simplicity a pair ofpoles is illustrated on opposite sides of the rotor, as indicated by theletters N and S on the drawings. Positioned in proximity to pole faces15 and 16 are shading rings 17 and 18, respectively. Shading rings 17and 18 delay flux applied by core 13 to rotor 12 so that the rotor canbe rotated in response to a reversal of the direction of flux flowingbetween pole faces 15 and 16.

Magnetic flux is applied in opposite directions selectively between polefaces 15 and 16 by center tapped winding or coil 19. Center tap 21 ofwinding 19 is connected to the positive terminal of D.C. power supply 22via power switch 23 and selectively feeds D.C. current to terminals 24and 25 at opposite ends of the winding. Control of the direction ofcurrent flow through winding 19 is determined by the conducting state ofNPN junction transistor 26 which functions effectively as a voltagecontrolled switch having a control electrode responsive to the level ofthe voltage applied to its base 27. The emitter 28 and collector 29 oftransistor 26 are connected in series circuit with the half of winding19 between center tap 21 and terminal 24, as well as with the voltage ofD.C. power supply 22. Base emitter junction bias to transistor 26 isprovided through the half of winding 19 connected between center tap 21and terminal 25 from the positive terminal of power supply 22 and basecurrent limiting resistor 31. The base emitter junction of transistor 26is selectively short-circuited by switch 32, connected between terminal25 'and the negative terminal of D.C. power supply 22. Switch 32 isnormally biased to an open circuit condition by spring 33 and ismomentarily closed in response to engagement thereof by cam 34 driven byrotating shaft 35.

With switch 32 in the normal open-circuited condition, the positivevoltage of DC power supply 22 is current flows from center tap 21 toterminal 24 and magnetic flux flows in a clockwise direction about core13 to electromagnetically maintain the north and south poles ofpermanent magnet rotor 12 in a locked or braked condition, with thesouth pole of the rotor adjacent pole face 16 and the north pole of therotor adjacent pole face 15.

When switch 32 is activated to the closed position in response tosuitable rotation of cam 34, the base 27 and emitter 28 are connectedtogether and a zero potential subsists between them. In response to azero potential existing across the emitter base junction of transmitter26, the transistor is activated to a cut-off condition and substantiallyan open circuit exists from the positive terminal of DC. source 22through the portion of winding 19 between center cap 21 and terminal 24.Simultaneously, the impedance in the circuit connecting the positive andnegative electrodes of DC. source 22 between center tap 21 and terminal25 of winding 19 is reduced substantially to a short circuit andsubstantial DC. current flows in the winding between the center tap andterminal 25. In response to the DC. current flowing between center tap21 and terminal 25, the direction of flux in core 13 is switched fromthe clockwise direction to the counterclockwise direction. In responseto the switching of flux from the clockwise to the counterclockwisedirection, rotor 12 is rotated; in the specific two pole embodimentillustrated, the rotor undergoes 180 of rotation. Rotor 12 rotates inresponse to the change in direction of flux between pole faces 15 and 16because shading rings 17 and 18 are provided in proximity to the polefaces. If the shading rings or some other equivalent type means were notprovided, the magnetic field would simply reverse in the air gap 14 withno stepping of rotor 12 being effected. After rotor 12 has been rotated180 it remains in situ due to the constant flow of magnetic flux throughit and between pole faces 15 and 16. Rotor 12 thereby remains locked inplace until the polarity of flux through air gap 14 is reversed again inresponse to switch 32 opening.

When switch 32 opens a positive bias is again applied to base electrode27 of transistor 26 and substantially a short circuit is establishedbetween the transistor emitter and collector electrodes wherebysufficient current again flows in the counterclockwise direction aboutcore 13 to step permanent magnet rotor 12 another 180. Rotor 12 iscontinuously braked in the stated position until switch 34 again closes.It is noted that rotor 12 is continuously braked and incrementallyrotated as required without the need for any permanent magnets in core13; further, the core includes only one air gap 14 to maintain the rotorin a braked position.

To suppress transients that might be induced across electrodes oftransistor 26 in response to sudden changes in current flowing throughwinding 19, diodes 36 and 37 are provided. Diodes 36 and 37 are normallyback biased by connecting the cathode of each of them to center tap 21and thence to the positive terminal of DC. power supply 22. The anode ofdiode 36 is connected to terminal 25, while the anode of diode 37 isconnected to terminal 24. Diodes 36 and 37 absorb positive currentflowing from terminals 24 and 25 while current is being switched inwinding 19 to provide short circuits about the windings and preclude thesubstantial flow of heavy transient currents from the windings to thebase and collector electrodes of transistor 26.

To minimize the DC. current applied by source 22 to winding 19 understeady state conditions while rotor 12 is electromagnetically braked,the circuit of FIG. 2 can be employed. In the circuit diagram of FIG. 2,as well as FIG. 3, there are illustrated only the circuit components, tothe exclusion of the motor core and rotor to simplify the presentation.In the circuit illustrated by the diagram of FIG. 2, the bias circuitfor the base emitter junction of transistor 26 is modified so that theDC. current through the emitter collector path of the transistor isreduced once rotor 12 has been rotated without substantially affectingthe current applied to winding 19 between center tap 21 and terminal 24while the rotor is being driven.

To these ends, a resistive voltage divider comprising resistors 41 and42 is connected between terminal 25 and the negative terminal of DC.source 22. The common junction between resistors 41 and 42 is connectedvia capacitor 43 to terminal 25 and via resistor 44 to base electrode27, of transistor 26. Base bias for transistor 26 is stabilized forsteady state conditions by resistor 45, connected between base 27 andemitter 28.

In steady state conditions, the DC. bias network including resistors 41,42, 44 and 45 maintains a relatively low forward bias across the emitterbase junction of transistor 26 so that the amount of current flowingthrough winding 19 between center tap 21 and terminal 24 is considerablyless than that required to step rotor 12. The current flowing throughwinding 19 between center tap 21 and terminal 24 under steady stateconditions, however, is adequate to maintain the rotor 12electromagnetically braked and preclude rotation thereof. In response toswitch 32 being closed, charge accumulated on capacitor 43 while switch32 was open circuited is dissipated by the path including resistors 42,44 and 45 so that transistor 26 is driven sharply into cut-off. Whilecapacitor 43 is being discharged, current flows from center tap 21 toterminal 25 through switch 32 to produce a flux in a direction oppositefrom the flux produced while switch 32 is open circuited. Because switch32 is normally short circuited for only short time durations, thiscurrent can be at a relatively high level without impairing efficiencyof the unit.

Cam 33 opens circuits switch 32 after rotor 12 has been stepped inresponse to the current flowing from center tap 21 to terminal 25. Inresponse to switch 32 being open circuited, a sudden decrease in thecurrent supplied to capacitor 43 occurs to suddenly forward bias thebase emitter junction of transistor 26. In response to the forward biassuddenly applied to the base emitter junction of transistor 26,substantially short circuited conditions are produced between emitter 28and collector 29 so that a large current is suddenly applied to winding19 between center tap 21 and terminal 24. In response to the relativelylarge current flowing between center tap 21 and terminal 24, a largeflux is produced in air gap 14 to step rotor 12 180. As time progresses,capacitor 43 is charged and the base emitter junction of transistor 26is forward biased to a lesser extent, to increase the impedance betweenemitter 28 and collector 29. In response to the increased emittercollector impedance of transistor 26 the current in winding19 betweencenter tap 21 and terminal 24 decreases, to reduce the required steadystate current in winding 19; the steady state current is, however, ofsufficient magnitude to hold rotor 12 in situ.

Substantially the same result as is obtained with circuit of FIG. 2 isperformed with the circuit of FIG. 3. In FIG. 3, the base bias circuitof transistor 26 is identical to that for the circuit illustrated byFIG. 1, but the emitter bias circuit for the transistor is modified. Inparticular, emitter 28 is connected to the negative terminal of DC.source 22 via a relatively large, current limiting resistor 46, shuntedby capacitor 47. While steady state conditions exist for rotor 12 inresponse to current flowing from center tap 21 to terminal 24 throughthe emitter collector circuit of transistor 26 a relatively largevoltage drop exists between the emitter 28 and the negative terminal ofsource 22 through the voltage drop across resistor 46, that is alsomaintained across capacitor 47. Thereby, under steady state conditions,the current in winding 19 between center tap 21 and terminal 24 is at arelatively low level, only sufficient to electromagnetically brake rotor12.

In response to switch 32 being closed, the base emitter junction oftransistor 26 is back biased so that current flowing from emitter 28 toresistor 46 and capacitor 47 is substantially precluded. Capacitor 47 isthereby discharged through resistor 46 to reduce the voltage on emitter28 substantially to the voltage at the negative terminal of source 22.While capacitor 47 is discharging through resistor 46, however, thepositive bias applied by the capacitor voltage to emitter 28 maintainstransistor 26 in a sharply cut-off condition and the magnetic fluxresulting from current flowing between center tap 21 and terminal ismuch greater than any flux which might be produced in response tocurrent flowing between center tap 21 and terminal 24 immediately afterclosure of switch 32. Thereby, sufficient force is applied to rotor 12to cause the rotor to he stepped 180.

In response to switch 32 again being open circuited forward bias issuddenly applied to base electrode 27 of transistor 26 to establishsubstantially a short circuit between emitter 28 and collector 29. Thesudden decrease in impedance level between emitter 28 and collector 29results in a sudden increase in current flowing from center tap 21 toterminal 24 through the emitter collector path of transistor 26. Theimpedance of capacitor 47 to the sudden increase in current flowing fromemitter 28 is relatively low so that the current flowing from center tap21 to terminal 24 is large enough to supply sufticient flux to core 13to enable rotor 12 to be stepped 180". The relatively large cur rentapplied between center tap 21 and terminal 24 decreases in magnitude ascapacitor 47 is charged; the current level is great enough toelectromagnetically lock rotor 12 in place.

While the highly efficient steady state flux applying circuits of FIGS.2 and 3 are illustrated specifically in conjunction with a circuithaving the basic configuration illustrated by FIG. 1, it is to beunderstood that the biasing principles illustrated in FIGS. 2 and 3 areapplicable to other modifications, of the type described infra.

Reference is now made to FIG. 4 of the drawings wherein there isillustrated a further embodiment of the present invention. In theembodiment of FIG. 4, the center tap winding of FIG. 1 is replaced witha pair of separate windings 51 and 52 mounted on core 13 in proximity topole faces 15 and 16, respectively. By mounting windings 51 and 52 inproximity to pole faces 15 and 16 greater efficiency in couplingmagnetic flux from the windings to rotor 12 is attained than with thecircuit of FIG. 1 wherein the flux producing winding is mounted on a legof the core remote from the pole faces.

A further modification of the circuit illustrated by FIG. 4 concernsconnecting winding 52 between the emitter 28 of transistor 26 and thenegative terminal of DC. source 22. By connecting winding 52 in theemitter circuit of transistor 26, rather than the collector circuitthereof, the voltage supplied to the winding is reduced withoutaffecting the current fed to the winding or the flux produced by thewinding.

Still another embodiment of the invention is illustrated by the circuitdiagram of FIG. 5. In FIG. 5, the center tap and split windings of theembodiments illustrated by FIGS. 1 and 4, respectively, are replaced bya single winding or coil 54 wound on a leg of core 13. Winding 54 isresponsive to current supplied thereto in opposite directions to produceoppositely oriented fluxes in air gap 14 to rotate rotor 12. Theoppositely directed currents flow through the entire length of winding54, thereby to increase the amount of flux applied to rotor 12 for eachturn in the winding so that the power requirements to brake and steprotor 12 are reduced.

To supply oppositely directed currents to winding 54, there is providedcircuit 55 having a pair of NPN transistors 56 and 57 and a pair ofcomplementary PNP transistors 58 and 59. The emitter collector paths oftransistors 56 and 58 are connected in series with each other across thepositive and negative terminals of D.C.

power supply 22 so that the emitters of the transistors have a commonconnection established through the.

parallel combination of resistor 61 and capacitor 62. The emittercollector paths of transistors 57 and 59 are connected in series acrossthe positive and negative electrodes of power supply 22 so that thecollectors of these transistors have a common connection. The emitter oftransistor 57 is connected through the parallel combination of resistor63 and capacitor 64 to the negative terminal of DC. power supply 22. Thebases of transistors 56-59 are connected through current limitingresistors 65 and 66 to the positive electrode of source 22, while thenegative electrode of the source is selectively connected to all of thetransistor bases via ngrmally open circuited switch contact 32 andresistor 6 e The positive voltage applied to the bases of transistors 56and 57 under normal operating conditions while switch 32 is opencircuited, maintains the emitter base junction of the NPN transistors ina forpath of transistor 56 into the left end, as illustrated in FIG. 5,of the winding and out of the right end of the winding through theemitter collector path of transistor 57. Under steady state conditions,the current flowing through winding 54 from left to right is sufficientto maintain rotor 12 in an electromagnetically braked or lockedcondition. Due to the biasing networks including resistors 61 and 63 andcapacitors 62 and 64, similar to the biasing circuit illustrated by FIG.3, the steady state current through winding 54 is maintained at arelatively low level but is switched during transient operation to asufficiently high level to produce sufficient flux in core 13 to rotaterotor 12.

In response to switch 32 being closed the voltage at the negativeterminal of DC. source 22 is applied to the bases of transistors 56-59to back bias the emitter base junctions of transistors 56 and 57, whileforward biasing the emitter base junction of transistors 58 and 59. Inresponse to transistors 56 and 57 being back biased, DC. current flowfrom the positive electrode of source 22 to the left end of winding 54ceases. In response to transistors 58 and 59 being forward biased, D.C.current flows from the positive terminal of DC. source 22 through theemitter collector path of transistor into the right side of winding 54,from the left side of the winding and through the emitter collector pathof the transistor 58.

The magnitude of the current supplied through the emitter collector pathof transistor 58 and 59 that flows from the right end of winding 54 issufficient to produce a flux having a magnitude great enough to rotaterotor 12. Rotor 12 is maintained in a locked condition in response tothe flux existing across air gap 14 in response to the current flowoccuring while transistors 58 and 59 are in a conducting state. Sinceswitch 32 is closed for only relatively short time durations, there isno need to limit the steady state current through transistors 58 and 59to optimize efficiency.

Reference is now made to FIGS. 6 and 7 of the drawings wherein there isillustrated a modification of core 13 obviating the need for fluxdelaying shading rings 17 and 18. In the embodiment illustrated by FIGS.6 and 7, rotational force is applied by the magnetic flux of core 13 tostep rotor 12 by providing extended pole face segments 71 and 72. Poleface segments 71 and 72 extend circumferentially about rotor 12 by anangle of approximately 120 from the center of the main portion of polefaces and 16. Each of pole face segments 71 and 72 has a lateral extentrunning in the direction parallel to the axis of rotor 12 less than thelateral extent of the main segments of pole faces 15 and 16, and therebyhave a smaller area than the main segments. Each of the auxiliary polefaces 71 and 72 preferably is aligned in a plane running at right anglesto the sheet, as viewed in FIG. 6, so that maximum coupling of fluxbetween them and rotor 12 occurs. In response to a flux reversal appliedto the pole faces, the extended pole segments 71 and 72 pull rotor 12 tostep the rotor 180. When steady state conditions are reached, themajority of the flux flows between the major pole face segments becauseof the larger area thereof than pole face segments 71 and 72 and anytendency of the rotor 12 to be llil stepped in response to flux flowingbetween the seg mented pole faces 71 and 72 is overcome by the flux inisstepped at a constant rate each time a flux reversal occurs, which maynot occur with the shading ring arrangement because of the flux delayassociated therewith. The embodiment of FIGS. 6 and-7 has the addedadvantage of being very easily fabricated in a laminated structure.

Still another embodiment for stepping a rotor controlled with thecircuit of the present invention is illustrated in FIGS. 8 and 9. In theembodiment of FIGS. 8 and 9 the shading ring construction need not beemployed, while the extended pole face arrangement can be employed ifdesired. In the embodiment of FIGS. 8 and 9, rotor 73 is provided with apair of major permanent magnet poles, indicated on the drawings by thelarge letters N and S, as well as a pair of minor permanent magnetpoles, indicated on the drawings by the' smaller letters n and s. Themajor and minor permanent magnet poles are illustrated as being mutuallyorthogonal so that they extend in a direction running parallel to therotor longitudinal axis coincident with shaft 74. The permanent magnetpoles of rotor 73, however, are not necessarily displaced from the majorpermanent magnet poles by 90. In fact, a 45 displacement betweenadjacent like polarity permanent magnet poles may be preferable becauseof the increased repulsive force which can be attained thereby duringstepping of the rotor.

In accordance with the embodiment illustrated in FIG. 8, the major andminor magnetic poles are formed by using a single rotor having the majorand minor permanent magnet poles impressed thereon by a specialmagnetizing fixture. In the alternative, a pair of rotor segments 75 and76, as illustrated in FIG. 9, can be employed to establish the major andminor permanent magnetic poles. Cylindrical segment 75 carries the majorpoles and has a face thereof in abutting relationship with a face ofcylindrical rotor segment 76. The two abutting faces of rotor segments75 and 76 are bonded together by any suitable means in such a mannerthat the north and south poles of segment 75 are suitably displacedrelative to the north and south poles of segment 76.

The embodiment of FIGS. 8 and 9 also obviates the need for shading ringsand the inherent flux delay disadvantages associated therewith. Hence,the same advantages as set forth with regard to FIGS. 6 and 7 areachieved with the embodiment of FIGS. 8 and 9.

In operation, the rotor of FIGS. 8 and 9 is stepped in response to fluxreversals applied between pole faces 15 and 16 due to the interaction ofthe minor permanent magnet poles with the changing flux. While the fluxis changing, it produces a force of greater magnitude than the steadystate force on minor permanent magnet pole pieces in rotor 73 to repelthe rotor minor pole pieces into alignment with the center of core polefaces 15 and 16.

In the two embodiments illustrated by FIGS. 6 9 for activating rotor 12the offset effect of the magnetic field of the rotor or core causes arepulsive force between the rotor and core poles to provide asignificant turning moment to the rotor. This can be seen by consideringthe flux flow in the arrangement of FIG. 8 wherein, under steady stateconditions, the flux in core 13 produces a south magnetic pole in poleface 15 and a north magnetic pole in pole face 16. The north and southpoles in pole faces 15 and 16 resulting from flux applied to core 13maintains rotor 73 in a braked or locked condition. In response to achange in the magnetic flux direction flowing between pole faces 15 and16, a north magnetic pole is produced in pole face 15, while a southmagnetic pole occurs in pole face 16. The north pole in pole face 15repels the minor permanent magnet north pole of rotor 73 to torque therotor in a first direction about its shaft. Similarly, the south pole inpole face 16 repels the minor south pole of rotor to torque the rotor insaid first direction so that the rotor is stepped. The torques producedby the major permanent magnets interacting with fluxes derived from polefaces 15 and 16 are oppositely directed so that the rotor is not steppedin response to them.

In accordance with another aspect of the invention, specificallyillustrated in FIGS. 10 12, the magnetic flux flowing through pole facesand 16 is maintained equal, even though unequal flux excitation of corelegs terminating in pole faces occurs. Excitation of pole faces 15 and16 in the embodiments of FIGS. 10 12 is in response to current beingsupplied to coils or windings 81 and 82 on legs 83 and 84 of core 13.Windings 81 and 82 can be center taped as illustrated if the circuits ofFIGS. 1 3 are connected thereto or the center tap can be eliminated ifthe drive circuit of FIG. 5 is employed. In the alternative windings 81and 82 can be separate and excited in the manner illustrated by FIG. 4.

Unequal flux excitation in legs 83 and 84 can occur if coils or windings81 and 82 having differing numbers of turns are wound on legs 83 and 84of core 13 in proximity with rotor 12, or because of differences orimbalances in reluctance paths of the core between points ofelectromagnetic excitation and pole faces 15 and 16. Flux flow throughthe pole faces 15 and 16 is maintained substantially equal despiteunequal excitation of windings 81 and 82 by placing a relatively lowresistivity loop or strap 85 over pole faces 15 and 16 in closeproximity to rotor 12. For purposes of simplicity, rotor 12 isillustrated as being of the type illustrated in FIGS. 8 and 9, but it isto be understood that the pole faces 15 and 16 can be modified asillustrated in FIGS. 6 and 7, or that shading rings can be alsoincluded.

Low resistivity loop or strap 85, preferably fabricated from a materialsuch as copper, aluminum or silver, comprises a ring 86 having a slot 87running parallel to the ring longitudinal axis and extending through theentire thickness of the ring. Circumferential slot 88, having an angularextent of slightly less than 180 divides ring 86 into a pair ofconducting segments. At the ends of slot 88, rectangular cut-outs 89 and90 are provided to enable pole faces 15 and 16 to extend through ring 86into close proximity with rotor 12. In addition to enabling closemagnetic coupling to be achieved between core 13 and rotor 12, thecut-outs 89 and 90 provide close magnetic coupling between the core andring 86 so that the ring can function in a manner similar to a turn of atransformer windmg.

If windings 81 and 82 are excited in such a manner that unequal magneticfluxes flow into and out of pole faces 15 and 16 leakage occurs in theair gap 14 between the pole faces and rotor 12. In accordance with thepresent invention compensation for the unequal flux flow in pole faces15 and 16 resulting from diverse flux flow excitation to pole faces 15and 16 is provided by strap 85. In response to unequal flux flow infaces 15 and 16 a current is induced in ring 86 due to the magneticlines of flux crossing ring 86 in the portion of air gap 14 between polefaces 15 and 16. In response to the current induced in ring 86 a flux isproduced in pole faces 15 and 16 to induce opposite polarity currents inwindings 81 and 82. The currents induced in the winding 81 and 82 are ofsuch polarity as to tend to equalize flux in legs 83 and 84.

To consider a specific example, assume that coil 81 has one turn, whilecoil 82 has two turns, that equal switching currents are supplied to thetwo coils and that strap is not included. Under such conditions, theA.C. flux about leg 84 is twice that about leg 83. The magneticpotential of the pole face 16 is much higher relative to theferromagnetic frame 13 than pole face 15 and while these pole faces aremagnetically coupled one to the other the leakage flux from face 16 toframe 11 and to surfaces of leg 83 other than pole face 15 aresubstantial. Under this circumstance, considerably more flux flows frompole 16 than through pole 15. In 7 accordance with the present inventionwherein strap 85 is provided, all of the A.C. flux flowing from pole 16while rotor 12 is being driven induces a voltage in strap 85. The A.C.flux through pole 15, due to the aforementioned difference in magneticpotential of it relative to pole 16 is less than the A.C. flux flowthrough pole 16, whereby the induced voltage at pole 15 is less than theinduced voltage at pole 16. The higher induced voltage at pole 16 causesA.C. current flow in a direction to increase A.C. flux flow in pole 15.

The low resistivity strap illustrated in FIG. 6 can also be employed inconjunction with a step motor having a magnetic core 91 shaped generallylike a wheel, with a pair of aligned spokes 95 and 97 connected togetherby annular outer rim 92. On spoke 95 is wound coil 96 which can becentered tapped, segmented or continu ous without a tap, but is excitedwith opposite polarity currents in accordance with a circuit of the typeillustrated by any of FIGS. 1 5.

Segmented spoke 97, comprising stub arms 98 and 99 and air gap 14,extends between the center of core 91 and rim 92 and is aligned withspoke 95. At the ends of arms 98 and 99 are located curved pole faces 15and 16 to form air gap 14 in which permanent magnet rotor 12 isdisposed. Ring 85, as illustrated in FIG. 12, flts on pole pieces 15 and16 in the same manner as-illustrated in FIG. 10 to couple magnetic fluxequally to both pole faces 15 and 16 of spoke 97 despite the differencein distance and magnetic reluctance between the two pole faces andexcitation coil 96. Hence, strap 85 forms basically the same function asdescribed supra with regard to FIGS. 10 12 in that unequal fluxexcitation in legs 98 and 99 generally occurs. While a circularconfiguration is illustrated in FIG. 13, it is to be understood that thecore could be of the square, rather than circular type, if lessefficient flux coupling can be tolerated.

While there has been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims.

l claim:

1. A circuit responsive to current derived from a DC source for drivinga step motor having excitation winding means and a permanent magnetrotor comprising voltage responsive switch means including a controlelectrode for selectively controlling a conducting path of the switchmeans to closed and open circuit conditions, a pair of contactsselectively connected together for selectively controlling bias voltageapplied to the control electrode to control the conducting state of theswitch means, and means for connecting in circuit with each other saidcontacts, the switch means conducting path, winding means and DC. sourceso that in response to the contacts not being connected together v theswitch means conducting path is closed and current flows from the sourcethrough the switch means conducting path to the winding means in a firstdirection and in response to the contacts being connected together theswitch means conducting path is open circuited and current flows fromthe source through the contacts to the winding means in a seconddirection, whereby the rotor is stepped each time the state of thecontacts is changed.

2. A circuit responsive to current derived from a DC. source for drivinga step motor having excitation winding means and a permanent magnetrotor comprising an electric discharge device including a controlelectrode and a pair of output terminals, said control electrode beingselectively forward biased in response to the conducting state of a pairof selectively open and closed contacts, whereby high and lowimpedancepaths exist through the discharge device between the output terminalswhile the contacts are respectively closed and open, means forconnecting said winding means and said closed contacts in series circuitwith the output terminals and DC. source so that current flows from theDC. source through the winding means in a first direction while the lowimpedance path exists, and means for connecting the winding means inseries circuit with the DC. source and closed contacts so that currentflows from the DC. source through the winding means in a seconddirection while the high impedance path exists. 3

3. In combination, a step motor having a'permanent magnet rotor, amagnetic core for coupling magnetic flux to said rotor, winding means onsaid core, circuit means for sequentially feeding oppositely directedcurrents to said winding means to induce oppositely directed fluxes insaid core and rotor, means in at least one of the core or rotor forstepping the rotor in response to each change in the direction of thefluxes in said core and rotor, said circuit means including two positionswitch means having first and second contacts selectively open and shortcircuited for controlling a DC. current continuously fed in only onedirection to the winding means while the first and second contacts areopen circuited and for controlling a DC. current continuously fed inonly the other direction to the winding means while the first and secondcontacts are short circuited, said D.C. currents being maintained untileach change in the current direction occurs and of sufficient magnitudeto induce a flux in the core and rotor to electromagnetically brake therotor, said D.C. currents reversing the flux direction each time thecurrent direction is changed, said circuit means further including meansfor supplying a current of greater amplitude to the winding means whilethe rotor is being stepped than the DC. current supplied to the windingmeans to brake the rotor.

4. The combination of claim 3 wherein said current supplying meansincludes: a voltage controlled impedance having a control electroderesponsive to the condition of said switch, and resistance capacitancecharging circuit means controlling the voltage magnitude applied to thecontrol electrode.

5. In combination, a step motor having a permanent magnet rotor, amagnetic core for coupling magnetic flux to said rotor, winding means onsaid core, circuit means for sequentially feeding oppositely directedcurrents to said winding means to induce oppositely directed fluxes insaid core and rotor, means in at least one of the core or rotor forstepping the rdtor in response to each change in the direction of thefluxes in saidcore and rotor, said circuit means including two positionswitch means having first and second contacts selectively open and shortcircuited for controlling a DC. current continuously fed in only onedirection to the winding means while the first and second contacts areopen circuited and for controlling a DC. current continuously fed inonly the other direction to the winding means while the first and secondcontacts are short circuited, said DLC. currents being maintained untileach change in the current direction occurs and of sufficient magnitudeto induce a flux in the core and rotor to electromagnetically brake therotor, said D.C. currents reversing the flux direction each time thecurrent direction is changed, said winding means including first andsecond segments, said circuit means further including: means for feedinga current in one direction to only said first segment during a firsttime interval and means for feeding a current in a direction opposite tothe first direction to only the second segment during a second timeinterval.

6. The combination of claim 5 wherein the core includes a pair of polefaces on opposite sides of the rotor, and further including a lowresistivity strap fitting over both said pole faces and interceptingleakage magnetic flux flowing from one pole face but not into the otherpole face, said strap having slot means so that current is inducedtherein by the intercepted leakage flux, said strap interfitting withthe pole faces so the current induced in the strap has a tendency tosubstantially equalize fluxes in the pole faces.

7. The combination of claim 6 wherein said winding means comprises apair of coils, each wound on a different leg feeding flux to the polefaces on opposite sides of the rotor.

8. The combination of claim 6 wherein said winding means comprises asingle coil wound on a leg feeding flux to the pole faces on oppositesides of the rotor, the magnetic reluctance of the core from the coil tothe two different pole faces being different.

9. The combination of claim 6 wherein the circuit means further includesresistance capacitance network means in a bias circuit for controlelectrodes of both transistors of one of the pairs of switches, saidresistance capacitance network means varying the impedance of thetransistor of said one of said switches so that the current suppliedbetween said ends is greater while the rotor is being stepped inresponse to current flowing between said ends than the current suppliedbetween said ends while the rotor is braked by the current flowingbetween said ends.

10. The combination of claim 5 wherein the circuit means includes asingle voltage controlled switch having a control electrode responsiveto the condition of the switch means and a pair of terminals connectedin series between a DC. source and one oftthe winding segments.

11. The combination of claim 5 wherein the other winding segment isconnected between one of the terminals and a control electrode of thevoltage controlled switch and in series between the DC. source and acontrol switch, said control switch being connected between the controlelectrode and other terminal of the voltage controlled switch.

12. The combination of claim 6 further including a resistancecapacitance circuit in a bias network for the control electrode of thevoltage controlled switch, said resistance capacitance circuit varyingthe impedance of the voltage controlled switch between said terminals sothat the current supplied to said one winding segment is greater whilethe rotor is being stepped in response to current flowing through saidone winding segment than the current supplied to said one windingsegment while the rotor is braked by the current flowing through saidone segment.

13. In combination, a step motor having a permanent magnet rotor, amagnetic core for coupling magnetic flux to said rotor, winding means onsaid core, circuit means for sequentially feeding oppositely directedcurrents to said winding means to induce oppositely directed fluxes insaid core and rotor, means in at least one of the core or rotor forstepping the rotor in response to each change in the direction of thefluxes in said core and rotor, said circuit means including twopositionswitch means having first and second contacts selectively open and shortcircuited for controlling a DC. current continuously fed in only onedirection to the winding means while the first and second contacts areopen circuited and for controlling a DC. current continuously fed inonly the other direction to the winding means while the first and secondcontacts are short circuited, said D.C. currents being maintained untileach change in the current direction occurs and of sufficient magnitudeto induce a flux in the core and rotor to electromagnetically brake therotor, said D.C. currents reversing the flux direction each time thecurrent direction is changed, the circuit means including: a first pairof voltage controlled switches for feeding current in one direction froma DC. source between said first and second ends, and a second pair ofvoltage controlled switches for feeding current in a second directionfrom the DC. source between said first and second ends; I and controlmeans for enabling both switches of the first pair while disabling bothswitches of the second pair or for disabling both switches of the firstpair while enabling both switches of the second pair, each switch ofsaid first pair of switches comprising a transistor of a firstconductivity type, each switch of said second pair of switchescomprising a transistor of a second conductivity type, said first andsecond conductivity types being opposite from each other, said controlmeans including said two-position switch means for selectivelyconnecting control electrodes of said transistors to be responsive tovoltages of opposite polarities derived from the DC. source, one contactof said switch means being directly connected to the control electrodesof all of said transistors.

14. The combination of claim 13 wherein the core includes a pair of polefaces on opposite sides of the rotor, and further including a lowresistivity strap fitting over both said pole faces and interceptingleakage magnetic flux flowing from one pole face but not into the otherpole face, said strap having slot means so that current is inducedtherein by the intercepted leakage flux, said strap interfitting withthe pole faces so the current induced in the strap has a tendency tosubstantially equalize fluxes in the pole faces.

15. The combination of claim 14 wherein said winding means comprises apair of coils, each wound on a different leg feeding flux to the polefaces on opposite sides of the rotor.

16. The combination of claim 14 wherein said winding means comprises asingle coil wound on a leg feeding flux to the pole faces on oppositesides of the rotor, the magnetic reluctance of the core from the coil tothe two different pole faces being different.

17. The combination of claim 14 wherein the slot means includes a firstslot extending longitudinally of the strap between the pole faces todivide the strap into first and second segments extending for the entiredistance between edges of the two pole faces, said first and secondsegments being joined after passing the edges of the two pole faces toform a pair of joining seg ments, said pair of joining segments beingseparated by a second slot in the strap.

18. In combination, a step motor having a permanent magnet rotor, amagnetic core for coupling magnetic flux to step the rotor in responseto changes in the direction of flux flowing from the core to the rotor,winding means on said core for inducing a, magnetic flux in the core,circuit means connected to said winding means for supplying current tosaid winding means to induce oppositely directed first and second fluxesin the core to rotate the rotor, said circuit means including: switchmeans having a voltage responsive control electrode for controllingcurrent flow between a pair of output terminals, means for selectivelyapplying a bias voltage to the control electrode to provide high and lowimpedance paths through the switch means between the pair of outputterminals, first circuit means connecting said pair of output terminalsin series with said winding means and a DC source for continuouslysupplying current to the winding means in a direction to induce a fluxin the rotor in the first direction while the low impedance path existsthrough the switch means, and second circuit means connecting saidwinding means and the DC. voltage source in series for continuouslysupplying current to the winding means in a direction to induce a fluxin the rotor in the second direction only while the high impedance pathexists second winding segment, said second circuit means including meansfor applying the bias voltage to the control electrode from the DC.source through the second winding segment.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 ,720,864 Dated March 13 1973 Inventor(s) Walter Kohlhagen It is certifiedthat error appears in the above-identified patent and that said- LettersPatent are hereby corrected as shown below:

On the title page the inventor s name should be corrected from"Kolhagen" to -Kohlhagen,

On each sheet of drawings the inventor's name should be corrected from"Kolhagen" to Kohlhagenv.

Claim 2, line 45, delete "and said closed contacts";

line 52, after "means" insert -and said closed contacts--.

Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents F ORM PO-105O (10-69) USCOMM-DC 60376-P69 fi U45 GOVERNMENTPRINTING OFFICE I969 0-'365-33A TJNITED STATES PATENT OFFICE CERTIFICATEOF CORRECTIQN Patent No. 3,720,864 Dated March 13, 1973 Inventor(s)Walter Kohlhagen It is certified that error appears in theabove-identified patent and that said- Letters Patent are herebycorrected as shown below:

On the title page the inventor's name should be corrected from"Kolhagen" to --Kohlhagen--.

On each sheet of drawings the inventor's name should be corrected from"Kolhagen" to -Kohlhagen--.

Claim 2, line 45, delete "and said closed contacts";

line 52, after "means" insert --and ,said closed contacts'-.

Signed and sealed this 12th day of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON ,JR. c. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM PO-IO USCOMM-DC 60376-P69 & 0.5 GOVERNMENT PRINTING OFFICE:1969 0-366-334

1. A circuit responsive to current derived from a D.C. source fordriving a step motor having excitation winding means and a permanentmagnet rotor comprising voltage responsive switch means including acontrol electrode for selectively controlling a conducting path of theswitch means to closed and open circuit conditions, a pair of contactsselectively connected together for selectively controlling bias voltageapplied to the control electrode to control the conducting state of theswitch means, and means for connecting in circuit with each other saidcontacts, the switch means conducting path, winding means and D.C.source so that in response to the contacts not being connected togetherthe switch means conducting path is closed and current flows from thesource through the switch means conducting path to the winding means ina first direction and in response to the contacts being connectedtogether the switch means conducting path is open circuited and currentflows from the source through the contacts to the winding means in asecond direction, whereby the rotor is stepped each time the state ofthe contacts is changed.
 1. A circuit responsive to current derived froma D.C. source for driving a step motor having excitation winding meansand a permanent magnet rotor comprising voltage responsive switch meansincluding a control electrode for selectively controlling a conductingpath of the switch means to closed and open circuit conditions, a pairof contacts selectively connected together for selectively controllingbias voltage applied to the control electrode to control the conductingstate of the switch means, and means for connecting in circuit with eachother said contacts, the switch means conducting path, winding means andD.C. source so that in response to the contacts not being connectedtogether the switch means conducting path is closed and current flowsfrom the source through the switch means conducting path to the windingmeans in a first direction and in response to the contacts beingconnected together the switch means conducting path is open circuitedand current flows from the source through the contacts to the windingmeans in a second direction, whereby the rotor is stepped each time thestate of the contacts is changed.
 2. A circuit responsive to currentderived from a D.C. source for driving a step motor having excitationwinding means and a permanent magnet rotor comprising an electricdischarge device including a control electrode and a pair of outputterminals, said control electrode being selectively forward biased inresponse to the conducting state of a pair of selectively open andclosed contacts, whereby high and low impedance paths exist through thedischarge device between the output terminals while the contacts arerespectively closed and open, means for connecting said winding meansand said closed contacts in series circuit with the output terminals andD.C. source so that current flows from the D.C. source through thewinding means in a first direction while the low impedance path exists,and means for connecting the winding means in series circuit with theD.C. source and closed contacts so that current flows from the D.C.source through the winding means in a second direction while the highimpedance path exists.
 3. In combination, a step motor having apermanent magnet rotor, a magnetic core for coupling magnetic flux tosaid rotor, winding means on said core, circuit means for sequentiallyfeeding oppositely directed currents to said winding means to induceoppositely directed fluxes in said core and rotor, means in at least oneof the core or rotor for stepping the rotor in response to each changein the direction of the fluxes in said core and rotor, said circuitmeans including two position switch means having first and secondcontacts selectively open and short circuited for controlling a D.C.current continuously fed in only one direction to the winding meanswhile the first and second contacts are open circuited and forcontrolling a D.C. current continuously fed in only the other directionto the winding means while the first and second contacts are shortcircuited, said D.C. currents being maintained until each change in thecurrent direction occurs and of sufficient magnitude to induce a flux inthe core and rotor to electromagnetically brake the rotor, said D.C.currents reversing the flux direction each time the current direction ischanged, said circuit means further including means for supplying acurrent of greater amplitude to the winding means while the rotor isbeing stepped than the D.C. current supplied to the winding means tobrake the rotor.
 4. The combination of claim 3 wherein said currentsupplying means includes: a voltage controlled impedance having acontrol electrode responsive to the condition of said switch, andresistance capacitance charging circuit means controlling the voltagemagnitude applied to the control electrode.
 5. In combination, a stepmotor having a permanent magnet rotor, a magnetic core for couplingmagnetic flux to said rotor, winding means on said core, circuit meansfor sequentially feeding oppositely directed currents to said windingmeans to induce oppositely directed fluxes in said core and rotOr, meansin at least one of the core or rotor for stepping the rotor in responseto each change in the direction of the fluxes in said core and rotor,said circuit means including two position switch means having first andsecond contacts selectively open and short circuited for controlling aD.C. current continuously fed in only one direction to the winding meanswhile the first and second contacts are open circuited and forcontrolling a D.C. current continuously fed in only the other directionto the winding means while the first and second contacts are shortcircuited, said D.C. currents being maintained until each change in thecurrent direction occurs and of sufficient magnitude to induce a flux inthe core and rotor to electromagnetically brake the rotor, said D.C.currents reversing the flux direction each time the current direction ischanged, said winding means including first and second segments, saidcircuit means further including: means for feeding a current in onedirection to only said first segment during a first time interval andmeans for feeding a current in a direction opposite to the firstdirection to only the second segment during a second time interval. 6.The combination of claim 5 wherein the core includes a pair of polefaces on opposite sides of the rotor, and further including a lowresistivity strap fitting over both said pole faces and interceptingleakage magnetic flux flowing from one pole face but not into the otherpole face, said strap having slot means so that current is inducedtherein by the intercepted leakage flux, said strap interfitting withthe pole faces so the current induced in the strap has a tendency tosubstantially equalize fluxes in the pole faces.
 7. The combination ofclaim 6 wherein said winding means comprises a pair of coils, each woundon a different leg feeding flux to the pole faces on opposite sides ofthe rotor.
 8. The combination of claim 6 wherein said winding meanscomprises a single coil wound on a leg feeding flux to the pole faces onopposite sides of the rotor, the magnetic reluctance of the core fromthe coil to the two different pole faces being different.
 9. Thecombination of claim 6 wherein the circuit means further includesresistance capacitance network means in a bias circuit for controlelectrodes of both transistors of one of the pairs of switches, saidresistance capacitance network means varying the impedance of thetransistor of said one of said switches so that the current suppliedbetween said ends is greater while the rotor is being stepped inresponse to current flowing between said ends than the current suppliedbetween said ends while the rotor is braked by the current flowingbetween said ends.
 10. The combination of claim 5 wherein the circuitmeans includes a single voltage controlled switch having a controlelectrode responsive to the condition of the switch means and a pair ofterminals connected in series between a D.C. source and one of thewinding segments.
 11. The combination of claim 5 wherein the otherwinding segment is connected between one of the terminals and a controlelectrode of the voltage controlled switch and in series between theD.C. source and a control switch, said control switch being connectedbetween the control electrode and other terminal of the voltagecontrolled switch.
 12. The combination of claim 6 further including aresistance capacitance circuit in a bias network for the controlelectrode of the voltage controlled switch, said resistance capacitancecircuit varying the impedance of the voltage controlled switch betweensaid terminals so that the current supplied to said one winding segmentis greater while the rotor is being stepped in response to currentflowing through said one winding segment than the current supplied tosaid one winding segment while the rotor is braked by the currentflowing through said one segment.
 13. In combination, a step motorhaving a permanent magnet rotor, a magnetic core for coupling magnetIcflux to said rotor, winding means on said core, circuit means forsequentially feeding oppositely directed currents to said winding meansto induce oppositely directed fluxes in said core and rotor, means in atleast one of the core or rotor for stepping the rotor in response toeach change in the direction of the fluxes in said core and rotor, saidcircuit means including two-position switch means having first andsecond contacts selectively open and short circuited for controlling aD.C. current continuously fed in only one direction to the winding meanswhile the first and second contacts are open circuited and forcontrolling a D.C. current continuously fed in only the other directionto the winding means while the first and second contacts are shortcircuited, said D.C. currents being maintained until each change in thecurrent direction occurs and of sufficient magnitude to induce a flux inthe core and rotor to electromagnetically brake the rotor, said D.C.currents reversing the flux direction each time the current direction ischanged, the circuit means including: a first pair of voltage controlledswitches for feeding current in one direction from a D.C. source betweensaid first and second ends, and a second pair of voltage controlledswitches for feeding current in a second direction from the D.C. sourcebetween said first and second ends; and control means for enabling bothswitches of the first pair while disabling both switches of the secondpair or for disabling both switches of the first pair while enablingboth switches of the second pair, each switch of said first pair ofswitches comprising a transistor of a first conductivity type, eachswitch of said second pair of switches comprising a transistor of asecond conductivity type, said first and second conductivity types beingopposite from each other, said control means including said two-positionswitch means for selectively connecting control electrodes of saidtransistors to be responsive to voltages of opposite polarities derivedfrom the D.C. source, one contact of said switch means being directlyconnected to the control electrodes of all of said transistors.
 14. Thecombination of claim 13 wherein the core includes a pair of pole faceson opposite sides of the rotor, and further including a low resistivitystrap fitting over both said pole faces and intercepting leakagemagnetic flux flowing from one pole face but not into the other poleface, said strap having slot means so that current is induced therein bythe intercepted leakage flux, said strap interfitting with the polefaces so the current induced in the strap has a tendency tosubstantially equalize fluxes in the pole faces.
 15. The combination ofclaim 14 wherein said winding means comprises a pair of coils, eachwound on a different leg feeding flux to the pole faces on oppositesides of the rotor.
 16. The combination of claim 14 wherein said windingmeans comprises a single coil wound on a leg feeding flux to the polefaces on opposite sides of the rotor, the magnetic reluctance of thecore from the coil to the two different pole faces being different. 17.The combination of claim 14 wherein the slot means includes a first slotextending longitudinally of the strap between the pole faces to dividethe strap into first and second segments extending for the entiredistance between edges of the two pole faces, said first and secondsegments being joined after passing the edges of the two pole faces toform a pair of joining segments, said pair of joining segments beingseparated by a second slot in the strap.